Method and apparatus for rapid thermal processing and bonding of materials using RF and microwaves

ABSTRACT

A method and apparatus for rapid and selective heating of materials using variable frequency RF and microwaves. The apparatus uses variable frequency solid state electronics as a microwave power source, a novel microwave heating head to couple microwave energy to the target materials and a match-up network to tune the frequency and impedance match between the microwave source and the load. An electronic and computer measurement and control system is employed to monitor and control the microwave heating process. The method teaches the use of inductive microwave coupling for thin conductive materials such as metal film and impurity doped silicon wafers. The method also teaches the use of capacitive microwave coupling for dielectric material such as glass and ceramics. The method further teaches the use of rapid and selective heating of heterostructure for bonding and sealing of mems and integrated circuits. The method and apparatus can provide ultra-high heating speed along with ultra-high heating temperatures for rapid thermal processing of semiconductors and other materials. It also allows the use of bonding materials with high melting temperature for strong bonding and sealing of mems and IC devices. The apparatus further provides for high interconnection density of integrated circuits as connections are made without the use of solder bumps.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Non-provisional applicationSer. Nos. 12/472,079 (currently pending) and 11/273,757 (now U.S. Pat.No. 7,569,800), filed May 26, 2009 and Nov. 15, 2005, respectively, andfurther claims the benefit of U.S. Provisional Application No.60/627,472, filed Nov. 15, 2004.

BACKGROUND

1. Field of Invention

The present invention relates to rapid thermal processing and bonding ofmaterials using radio frequencies (RF) and microwaves, and morespecifically to a method and apparatus for rapid heating of materialsusing solid state variable frequency RF/microwave sources. The inventionfurther relates to a method for rapid thermal processing ofsemiconductors and materials using the present RF/microwave apparatus.The present invention also relates to a method of selectively heatingportions of materials using the RF/microwave apparatus of the invention,and more specifically to the bonding and sealing ofmicroelectro-mechanical systems (mems) and integrated circuits(IC/microchips) using the present method of RF/microwave rapid andselective heating.

2. Related Art

Advantages of Rapid RF/Microwave Heating

There are many applications for using microwave energy for thermalprocessing of materials. Microwave heating has an advantage of rapidheating over conventional heating techniques. Conventional heating isslow because the heat is applied to the surface of an object. It takestime to transfer the heat from the surface to the interior of the objectthrough thermal conduction. In contrast, microwave heating is fastbecause microwave can penetrate into objects and heat the interior andexterior simultaneously.

Microwave-Materials Interaction

The mechanisms of microwave heating are based on the interaction betweenthe electromagnetic field and the targeted materials. There are threeforms of interaction in general. namely: (1) materials with low complexdielectric constant are poor microwave absorbers and not good formicrowave heating; (2) materials with high complex dielectric constantare strong microwave absorber and very good for microwave heating; and,(3) conductors such as metals reflects microwave so not good formicrowave heating in general. However the magnetic filed of microwavescan penetrate a skin depth of the conductor's surface and induce an eddycurrent to generate heat therein. The skin depth of high conductivemetals such as Au, Ag and Cu is about 0.6 to 6 μm within RF/microwavefrequency range from 100 MHz to 10 GHz. Therefore when the dimension ofa conductor becomes the same scale or less of the skin depth, such as inthe cases of the conductive interconnection in IC chips and dopedimpurity layer in Si wafers, the heating effect of eddy current issignificant and the thin conductor layer is no longer a microwavereflector but a strong microwave absorber. The thin layer of a conductorbecomes a very good candidate for microwave heating.

Selective Heating

When microwave power radiates on a heterostructure (comprised ofdifferent materials), such as a strong microwave absorber and a poormicrowave absorber, microwaves will selectively heat up the strongmicrowave absorber while leaving negligible heating effect on thematerials of low microwave absorption.

Methods and Apparatus for Microwave Rapid Heating

It is known that the heating rate of RF/microwave thermal processesdepends on the density of RF/microwave energy to be coupled into thematerials. Therefore two important conditions for a rapid RF/microwaveheating are (1) a high intensity of electromagnetic (EM) field and (2)EM energy to be efficiently coupled into the targeted materials. Themethods for coupling EM energy into materials can be characterized asthree classes:

1. Capacitive coupling, where energy is coupled into materials viaelectric E fields;

2. Inductive coupling, where energy is coupled into materials viamagnetic H fields: and,

3. Cavity (EH) coupling where energy is coupled into materials via acombination of E and H fields.

Cavity coupling is the most widely used method in microwave heatingprocesses. A microwave heating furnace is typically constructed in theform of either a single mode cavity or a multi-mode cavity operating ata fixed frequency. A single mode cavity can generate a much higherintensity of electromagnetic filed than that of a multi-mode andtherefore is more favorable in fast heating processes. Heating rates ashigh as 10-100° C./sec are achievable using single mode cavity whereasthe heating rate in a multi-mode cavity is relative low. There are sometechnical barriers for further increasing the heating rate to well above100° C./sec level using cavity techniques used in prior art. First theuse of a fixed frequency source leads to a mismatch in resonantfrequencies between the RF/microwave source and the loaded cavity whenthe frequency of the loaded cavity shifts with the temperature changesduring a thermal process. Second, the cavity is mechanically tuned soits response to coupling change is slow which results in a slow down ofthe heating rate.

There are other limitations of cavity coupling techniques in the priorart. One of the significant problems is the arcing and the breakdown ofplasma inside the cavity as the input RF/microwave power reaches athreshold level to breakdown the air. Especially in the presence ofconductive materials in the cavity, such as metal, the electric field issignificantly enhanced at the edge of conductive materials so the arcingmay occur at a much lower power level than that of the threshold. Thelimitation of input power due to the arcing problem significantly limitsthe ability of using cavity techniques to achieve high heating rate andhigh heating temperature.

Another limitation of cavity techniques used in the prior art is thatthe size of the load must be smaller than the size of the cavity. Sincethe dimension of a single mode cavity decreases with an increase of theoperating frequency, the size of the cavity operating at high frequencymay not be large enough when the size of load is big, e.g. an 8″ siliconwafer.

Induction coupling method is known in prior art and has been widely usedin heating conductive materials. The mechanism of the inductive heatingis based on the thermal effect of ‘eddy current’ induced by the magneticfield of an AC current. However, the use of induction heating in priorart is limited in low frequency levels, ranged from 50 Hz to less than50 MHz. None of the induction heating methods in the prior art has usedRF and microwave frequency beyond 100 MHz. Induction heating in notsuitable in heating low conductive materials and insulators due to thelack of eddy current.

3. Applications

Rapid Thermal Processing (RTP) of Semiconductors and Materials

An important application of microwave rapid heating techniques is rapidthermal processing (RTP) of advanced materials and semiconductors suchas activation annealing of ion implanted wafers for SiC and GaN widebandgap semiconductors and formation of ultra shallow junction for CMOSdevices. High annealing temperature and extremely short annealing timeare two critical conditions for these RTP applications. By increasingthe annealing temperature while simultaneously reducing the time, highpercentage of electrical activation can be achieved and will lead to lowsheet resistance. Meanwhile, the short annealing time will minimize thediffusion in CMOS devices to allow ultra shallow junction formation andeliminate surface evaporation in SiC wafers to reduce surface roughness.For instance, the temperature required for post-implant annealing of SiCsemiconductor may range from 1500 to 2000° C. along with short annealingtime ranged from a few minutes to a few seconds. None of conventionalheating methods used in prior art can meet these requirements. Limitedby the slow heating rate, conventional furnace heating methods can onlyperform SiC annealing at temperatures below 1700° C. with annealing timelonger than 5 minutes. The relative low annealing temperature of below1700° C. will result in ineffective recovery of crystal and lowactivation efficiency. The halogen lamp based commercial RTP systems canonly operate at below 1300° C. which makes them incapable for postimplant annealing of SiC.

Very high heating and cooling rates ranged from 100° C./sec to wellbeyond 1000° C./sec are required for RTP of CMOS shallow junction withintemperature range of 1150 to 1350° C. The halogen lamp based commercialRTP methods are currently used for RTP of CMOS shallow junction whichcan operate at heating rate of 100-300° C./sec within annealingtemperatures below 1300° C. The best current lamp based RTP systemsappear to be viable to shallow junction processes for current 100 nmtechnology node, although they may be very close to the limit of theircapability. However, they will no longer meet the challenges in makingultra-shallow junctions associated with next generation CMOS in sub-65nm regime or smaller where ultra-high heating rates well above 1000°C./sec are required.

Bonding and Packaging of Mems and IC Devices

The packaging of mems and IC chips generally requires the bonding of thecomponents and the connecting of the internal circuits with externalpins. The soldering method is a the most widely used technique for ICand mems packaging where low melting alloys such as Pb—Sn are used asthe solder materials. FIG. 1 illustrates a typical example of Flip-Chipsolder bonding in connecting the microchip to a substrate where thesubstrate may be either a print circuit board (PCB) or direct chipattachment (DCA). Two gold pads are first plated on the bonding areas ofthe silicon chip and substrate. The solder material is then placedbetween the two pads. By melting the solder to form a metal bump betweenthe pads as shown in FIG. 1, the silicon chip and the substrate arebonded together.

As the demands on miniaturization of IC chips increases, high-densityinterconnections and a high number of I/O requirements are indispensablein the IC industry. The large space occupied by the solder bumps becomesa limited fact for further shrinkage of the IC package. Another problemwith the soldering method is the high residual stress at the bondinginterfaces caused by the large coefficient of thermal expansion (CTE)mismatch between the solder bump and the substrates. The high residualstress may induce delamination and cracks in the package during theoperation in thermal cycles.

Many mems applications may require special bonding qualities such ashigh bonding strength, hermetic sealing, chemical resistance and highservice temperature. Bonding strength is very important for themechanical function of many mems. Mems components may also operate underdifferent environment, temperature and vacuum/pressure conditions wherehermetic sealing, chemical resistance and high service temperaturebecome the necessary bonding requirements. Low temperature solderbonding can no longer meet these requirements because of its weakmechanical strength, low melting temperature and poor chemicalresistance. Other bond techniques and bonding materials are needed. Forinstance, some noble metals such as Au, Cr, Ni, Pt or glass and ceramicsare very good candidates as the bonding materials because of theirexcellent high temperature properties and chemical resistance. Thesematerials require much higher bonding temperature than that of solderingbecause of their high melting temperature.

A number of bonding techniques have been developed for using bondingmaterials other than lower temperature solders, such as Au—Si eutecticbrazing, glass-Si fusion bonding and anodic bonding. These bondingtechniques have significantly improved the mems bonding quality to meetthe special requirements of various applications. However, all theseexisting bonding techniques are global heating methods so entire memsare exposed to high temperatures for long processing times which maylead to some damages or changes of the microcircuits and microstructurewhich were already placed on the mems components.

Thus it is important to develop alternative approaches to overcome theshortcomings of the existing techniques for these applications. What isneeded is a method for rapid heating up of materials to very hightemperatures for RTP of materials and semiconductors. What is alsoneeded is a method for strong bonding and hermetic sealing of mems withbonding materials of high melting temperatures or chemical resistance.What is further needed is a method for small size bonding of IC chipswithout using solder bumps.

SUMMARY

To overcome the limitations in the prior art described above, thepresent invention discloses a method and apparatus for rapid andselective heating of materials using variable frequency RF/microwavesources. The method and apparatus can provide ultra-high heating speedalong with ultra-high heating temperatures for RTP of materials thatother RTP techniques can not perform. It also will provide a newapproach to meet the challenges of high interconnect density ofintegrate circuits and high bonding quality for mems applications.

The primary objective of the present invention is to disclose aRF/microwave apparatus which is capable of rapid and selective heatingof materials to very high temperature. The heating rate depends on howefficiently the RF/microwave energy is coupled into the targetedmaterials. The RF/microwave apparatus of the present invention has anumber of advantages which overcome the limitation of prior art inmicrowave heating techniques and significantly improves the efficiencyof microwave coupling.

Another objective of the present invention is to teach a method forrapid thermal processing of semiconductors and materials using anRF/microwave apparatus. Experiments based on this method for rapidheating of SiC wafers and Arsenic (As) doped Si wafers have demonstratedthat the RF/microwave apparatus of this invention has the capability ofachieving ultra-high temperature of over 2000° C. and an ultra-fastheating rate of 2,000° C./second.

Another objective of the present invention is to teach a new approach ofbonding and sealing of mems and IC devices using RF/microwave rapid andselective heating apparatus of this invention. This new approach willallow the use of bonding materials of high melting temperature orchemical resistance such as noble metals, for strong bonding andhermetic sealing of mems and IC devices. The new approach also willeliminate the solder bumps in flip-chip and BGA bonding of IC devicesand as a result, achieve significantly higher interconnect density.

According to one aspect of the invention, inductive coupling andcapacitive coupling methods are used to replace the cavity couplingmethod in prior art to transfer RF/microwaves from the source to theload. The inductive coupling is good for thin conductive materials suchas metal film or impurity doped silicon wafers where the magnetic fieldwill induce eddy current to heat the conductive materials. Thecapacitive coupling is good for dielectric materials such as glass andceramics where the electric field will heat the high dielectric lossmaterials.

The use of inductive coupling or capacitive coupling methods in theheating head of the RF/microwave apparatus of the present invention isone of the key factors in providing many advantages over the cavitycoupling method used in the prior art. These advantages are: (1)different coupling mechanisms are used for different materials so thecoupling efficiency is optimized; (2) the RF/microwave head can beclosely placed on the top of the targeted heating area so the heatingareas are exposed to the strongest electromagnetic filed; (3) theconfiguration of the conductive coil or capacitive plates in theRF/microwave heating head can be specially designed to match with thepattern of the heating areas so intensive RF/microwave energy will onlybe delivered to the focused areas while no other areas will be affectedby the RF/microwave radiation; (4) there is no arcing or plasmabreakdown problem which means the inductive coupling method can deliversignificantly higher power density to the load than the cavity couplingmethod can. As a result, significantly higher heating rates and higherheating temperatures can be achieved); and, (5) there is no limitationon the size of the load

According to another aspect of the invention, variable frequency solidstate electronics, instead of a fixed frequency magnetron, are used asthe RF/microwave power source in the present invention. The variablefrequency source allows a broad bandwidth from which to choose theoptimal operating RF/microwave frequency. It also provides theflexibility of sweeping the source frequency during the thermalprocesses to compensate the resonant frequency shift of the load causedby the temperature change of the specimen. In addition, the variablefrequency source has the capability of generating pulsed or modulatedRF/microwave power for optimal energy dissipation. In comparison withthe traveling wave tube (TWT) variable frequency source used in priorsystems, the present solid state electronic source is much cheaper, morecompact and easier for electronic control.

According to another aspect of the invention, a match-up network isspecially designed for tuning both resonant frequency and impedance ofthe loaded RF/microwave heating head to match with those of theRF/microwave power source. The L and C parameters can vary with thetargeted specimens in order to maintain excellent coupling conditionsfor different applications. In contrast to the slow mechanical tuning ofmicrowave cavities in the prior art, the present L and C components inmatch-up network can be electronically tuned so that there is no delayin tuning response time and a much higher heating rate can be achieved.

According to another aspect of the invention, a network analyzer and PCcomputer are introduced in the measurement system for monitoring, tuningand controlling the entire microwave and thermal processes. Since therapid thermal processes must be completed in a very short time, e.g. afew seconds or less, it is very hard to tune and to control theprocesses manually. The combination of hardware and software realizes anautomated measurement and control system with real-time feedback, whichprovides further flexibility, stability and reliability to the overallsystem.

According to another aspect of the invention, microwave rapid heatingtests for SiC wafers was performed using the RF/microwave apparatus ofthis invention. Microwave power at a frequency of 402 MHz was applied tothe target specimen of SiC wafers through an inductive coil, which wasclosely placed on the top of the targeted specimen. It took about 1 and4 seconds for a SiC specimen to reach the temperatures of 1400° C. and2200° C. respectively with 150 W input power. The average heating ratevaries from 1400° C./sec to 550° C./sec depending on the targettemperature. Higher heating rates are achievable with the presentinvention.

According to another aspect of the invention, microwave rapid heating ofArsenic (As) doped Si wafers have been performed using the RF/microwaveapparatus of this invention. Microwave power with continuous sinewaveform at a frequency of 406 MHz was applied to the target of As dopedSi wafer through an inductive coil. It only took one second to heat theAs doped Si wafer to its melting temperature of 1410° C. at power levelof 150 W. The heating rate is about 1400° C./sec. The heating rate maybe significantly increased if high power pulsed microwave sources can beused that will have great potential in meeting the challenges of RTP fornext generation CMOS ultra-shallow junctions.

According to another aspect of the invention, rapid and selectiveheating of a heterostructure material of gold thin film coated siliconsubstrates is achieved using the method and apparatus of the invention.The heterostructure is formed by depositing Cr and gold thin films onthe silicon substrate. Microwave power at a frequency of 906 MHz isapplied to the target specimen through an inductive coil. The magneticfield of microwave power will induce a high eddy current in conductivelayer of gold thin films and rapidly heat up the gold thin film to overits melting point of 1064° C. in 2-3 seconds with input microwave powerof 120 to 140 Watts. Meanwhile the bulk of silicon substrate remainscool because of the fast heating speed and short heating time. Theresults of the tests demonstrate the capability of the RF/microwaveapparatus of the present invention in selective heating heterostructurematerials to high temperature at very high heating rate.

According to another aspect of the invention, two components of a memsmicro-pump are rapidly bonded using the method and RF/microwaveapparatus of the present invention. Gold thin films are deposited on thepatterned bonding areas of the silicon substrates using PVD sputteringand lithographical methods. The two silicon substrates are stacked upwith the gold coated areas face to face along with the alignment of thebonding areas. The two stacked substrates form a heterostructure wherethe gold is a strong microwave absorber and the silicon substrate is aweak microwave absorber. Microwave power of 904 MHz is coupled to thebonding target through an inductive coil. Since the silicon substrate isa low dielectric loss material and a weak microwave absorber, the inputmicrowave power can penetrate through the top silicon substrate anddissipated in the thin gold layers in the middle of the sandwiched memscomponents through an induced eddy current. It takes only about 2-3seconds to firmly bond the two components A and B of the mems micro-pumpwith microwave power level of 120-140 watts.

According to another aspect of the invention, a new approach foreliminating solder bumps in flip-chip and BGA bonding of IC chips hasbeen proposed by directly bonding gold pads on chip to gold pads forpins using the apparatus of the present invention. An induction head canbe placed on the top of IC microchip and eddy currents will be inducedin the gold coated pads. The two pads will be bonded when the goldmelting temperature of 1065° C. is reached. The heating rate must bevery fast so the bonding can be completed in a very short time. Only theinterface of the pads is melted while all other parts of the ICmicrochip are still cool and the microcircuits in the IC chip arepreserved and unaffected. The elimination of the solder bumps will allowmore extra space to be used to allocated more pins for higherinterconnect density.

The method and RF/Microwave apparatus of the present invention mayintroduce several new opportunities for bonding and packaging of memsand IC devices, including: (1) Much higher temperatures can be appliedto improve the bonding strength; (2) New bonding mechanisms such asbrazing and fusion bonding, and new bonding materials, such as noblemetals of Au, Cr, Ni, Pt and SiO₂ based glass can be used; (3) Nopressure needs to be applied to the specimens during bonding which cansignificantly reduce the residual stress related damages; and, (4)Bonding is not limited at the outside surface of the body. Microwavescan penetrate into non-conductive or low conductive materials to performbonding inside the body as demonstrated in the bonding of sandwichedmems components described above. (5) A new direct gold pad to gold padbonding method for solderless Flip-Chip and Ball Grid Array (BGA)Bonding of IC Chips to achieve significantly higher interconnect density

BRIEF DESCRIPTION OF THE DRAWINGS

The invention of the present application will be described in detailwith reference to the accompanying drawing(s), given only by way ofexample, in which:

FIG. 1 shows a traditional (prior art) flip chip bond with solder bumps;

FIG. 2 is a block diagram of the present RF/microwave rapid heatingapparatus;

FIG. 3 shows the set-up of the RF/microwave apparatus for rapid heatingof target materials;

FIG. 4 shows the inductive coils and capacitive electrodes used inRF/microwave heating head coupler;

FIG. 5 is an equivalent circuit diagram of the heating head;

FIG. 6( a) is a Smith chart graph for reflective coefficient and compleximpedance displayed on the screen of a Network Analyzer before tuning(not in an optima coupling);

FIG. 6( b) is a Smith chart graph for reflective coefficient and compleximpedance after tuning (optimal coupling is achieved);

FIGS. 7( a)&(b) are graphs showing the heating rate of SIC and siliconwafers achievable with the present apparatus; and,

FIG. 8 shows heating rate charts of SiC and silicon wafers that areachievable with the present apparatus.

FIG. 9 is a gold thin film coated silicon wafer;

FIG. 10 shows the configuration and dimensions of an inductive coil forrapid heating tests;

FIG. 11 is a graph that shows the RF/microwave rapid heating ofheterostructure of a gold coated silicon substrate heterostructure;

FIG. 12 shows heating rate tables for silicon substrates that areachievable with the present apparatus;

FIG. 13 shows RF/microwave rapid bonding of two mems components, wherein(a) is a cross section of a mems micro-pump; (b) shows the pattern ofthe bonding areas for the components; (c) shows PVD sputteringdeposition; and, (d) shows the stacking-up and alignment of thecomponents for RF/microwave rapid bonding;

FIG. 14( a) shows a traditional (prior art) flip chip bonding withsolder bump;

FIG. 14( b) shows solderless RF/microwave direct bonding of small goldpads for high IC interconnect density;

DETAILED DESCRIPTION OF THE INVENTION

RF/Microwave Apparatus for Rapid and Selective Heating of Materials

One objective of the present invention is to disclose a RF/microwaveapparatus which is capable of rapid and selective heating of materialsto very high temperature. FIG. 2 is a block diagram of the RF/microwaveapparatus for rapid heating of materials according to the presentinvention. This RF/microwave apparatus can be divided in to three mainparts: the variable frequency RF/microwave power source 1; theRF/microwave heating head 2; and, the measurement and control system 3.RF/microwave power is generated from a variable frequency power source1, which is driven by solid state electronics and coupled into a targetspecimen 4 through a RF/microwave heating head 2. A measurement andcontrol system 3, which is placed between 1 and 2, is used for tuning,monitoring and controlling of the RF/microwave thermal processes.

Solid State High Power RF/Microwave Sources

Still referring to FIG. 2, the variable frequency RF/microwave powersource 1 consists of a RF/microwave signal generator 11 and aRF/microwave power amplifier 12. High frequency signal is generated fromthe signal generator 11 and amplified by a RF power amplifier 12. Thepower amplifier 12 may be either a solid state RF power amplifier or atraveling wave tube (TWT) amplifier. The power source can sweep acertain bandwidth within a frequency range from 100 MHz to 20 GHz.Preferably, the driving frequency ranges from 300 MHz to 6 GHz. Theoutput power is adjustable from 0 to several hundred watts and up.

Microwave Heating Head and Match-Up Network

Still referring to FIG. 2, the RF/microwave heating head 2 consists of amatch-up network 22 and a heating head coupler 21. The heating headcoupler 21 may be either an inductive coil or a pair of capacitiveelectrodes. RF/microwave power generated from the power source 1 isforward to the RF/microwave heating head coupler 21 through adirectional coupler 33, a circulator 34, and a match-up network 22. Theheating head coupler 21 is closely placed on the top of the targetedspecimen 4 which is held and moved by a three dimensional stage (shownin detail in FIG. 3). The specimen is separated from the stage by aninsulator plate to minimize the thermal loss.

RF/microwave energy can be dissipated to the target specimen through theheating head coupler using either an inductive coupling method or acapacitive coupling method. The inductive coupling method is preferredin heating high conductive thin film such as metals or highly dopedimpurity layers in silicon wafers. The capacitive coupling method ispreferably to be used for materials with high dielectric loss of ∈″ suchas glass and ceramics.

The structure and configuration of the RF/microwave heating head couplerneeds to be specially designed according to the shape, dimension andelectric properties of target materials for achieving optimal couplingefficiency and high heating rate. Examples of inductive coils andcapacitive electrodes used in the RF/microwave heating head coupler inthe present invention are illustrated in FIG. 4. The shape and size ofthe coils may vary with the shape and size of the targeted area suchthat the best match and an optimal coupling efficiency can be achieved.The number of coil turns is preferably one turn or less when the drivingfrequency is higher than 500 MHz. The coil may have two turns or lesswhen the driving frequency is between 100 MHz to 500 MHz. In general,the effective electrical length of the coils is less than about ¼ thewavelength of the driving frequency applied. More preferably, theeffective electrical wavelength is less than 1/10 of the wavelength.

The match-up network 22 is used to tune both resonant frequency andimpedance of the loaded RF heating head to match with those of theRF/microwave power source. An equivalent circuit diagram of the heatinghead of the present invention is shown in FIG. 5, where L and R are theinductance and resistance of an inductive coil, respectively. C1 and C2are two capacitive elements of a split capacitor network in the match-upnetwork. They are used to tune both resonant frequency and impedance ofthe loaded inductive coil to match with those of the RF/microwave powersource. A match of the resonant frequency and impedance between theRF/microwave source and the loaded heating head circuits is crucial forachieving rapid heating. The optimal design of a match-up network varieswith applications. The selection of the component parameters of L, C1and C2 depend on both the properties of materials and dimensions of theload specimens. The resonant frequency ω₀ of this heating head can becalculated from Equation (1):ω₀ ²=1/L(1/C ₁+1/C ₂)  (1)Control and Measurement System

Referring again to FIG. 2, the measurement and control system consistsof a directional coupler 33, circulator 34, RF probes 35, power meters36, optical temperature measurement device (pyrometer) 37, NetworkAnalyzer 31 and a PC computer 32. The directional coupler 33 and RFprobes 35 are used to detect the transmitted and reflected microwavepower. The detected signals are sent to RF power meters 36 and amicrowave network analyzer 31 for monitoring the coupling of microwavesto the target materials. The optical pyrometer 37 is used to record thetarget temperature. All microwave and temperature data are input into aPC computer 32 where the measured parameters are instantly displayed astime dependent functions on the computer monitor.

Key Advantages of RF/Microwave Rapid Heating Apparatus

It is known that the heating rate depends on how efficiently theRF/microwave energy can be coupled into the targeted materials. TheRF/microwave apparatus of the present invention has the following keyadvantages to overcome the limitation of prior art in microwave heatingtechniques.

According to one aspect of the invention, inductive coupling andcapacitive coupling methods are used to replace the cavity couplingmethod of the prior art to transfer RF/microwaves from a source toloaded specimens. The inductive coupling is good for thin conductivematerials such as metal film or impurity doped silicon wafer where themagnetic field will induce an eddy current in the thin metal film toheat it rapidly. The capacitive coupling is good for dielectricmaterials such as glass and ceramics where the rapid heating is inducedby the electric field.

The use of inductive coupling and capacitive coupling in theRF/microwave head of the present invention is one of the key factors toproviding many advantages over the cavity coupling method used in priorart. These advantages are: (1) different coupling mechanisms are usedfor different materials so the coupling efficiency is optimized; (2) theRF/microwave head can be closely placed on the top of the targetedheating area so the heating areas are exposed in the strongestelectromagnetic filed; (3) the configuration of the conductive coil orcapacitive electrodes in the RF/microwave head coupler can be speciallydesigned to match the pattern of the heating areas so intenseRF/microwave energy is only delivered to the focused areas while noother area will be affected by RF/microwave radiation; (4) there is noarcing or plasma breakdown problem, which means the inductive couplingmethod can deliver significantly higher power density to the load thanthe cavity coupling method can. As a result, significantly higherheating rate and higher heating temperature can be achieved; and, (5)there is no limitation on the size of the load.

According to another aspect of the invention, variable frequency solidstate electronics, instead of a fixed frequency magnetron, are used asthe RF/microwave power source in the present invention. This allows abroad range of bandwidths from which to choose the optimal operatingRF/microwave frequency. It also provides the flexibility of sweeping thesource frequency during the thermal processes to compensate the resonantfrequency shift of the load caused by the temperature change of thespecimen. In addition, the variable frequency source has the capabilityof generating pulsed or modulated microwave power for optimal energydissipation. In comparison with the TWT variable frequency source usedin prior systems, the solid state electronic source is much cheaper,more compact and easier for electronic control.

According to another aspect of the invention, a match-up network isspecially designed for tuning both resonant frequency and impedance ofthe loaded heating head to match with those of the RF/microwave powersource. The L and C parameters for the components in the heating headcan vary with the targeted specimens in order to maintain excellentcoupling conditions for different applications. In contrast to the slowmechanical tuning of RF/microwave cavities in the prior art, the L and Ccomponents can be electronically tuned so that there is no delay intuning response time and a much higher heating rate can be achieved.

According to another aspect of the invention, a network analyzer and PCcomputer are introduced in the measurement system for monitoring, tuningand controlling the entire RF/microwave and thermal process. Since therapid thermal processes must be completed in a very short time, e.g. afew seconds or less, it is very hard to tune and to control theprocesses manually. The combination of hardware and software realizes anautomated measurement and control system with real-time feedback, whichprovides further flexibility, stability and reliability to the overallsystem.

Tuning RF/Microwave Apparatus to an Optimal Coupling Condition.

Since the heating rate depends on how efficiently the RF/microwaveenergy can be coupled into the materials. The RF/microwave system mustbe tuned in excellent match conditions for both the resonant frequencyand the impedance between the RF/microwave source and the load. This isaccomplished by a small signal test before applying high RF/microwavepower to run the rapid heating process. Referring to FIG. 2, in a smallsignal test, the signal generator 11 is operating in a sweep oscillatingmode with a low level signal output, e.g. 1 dbm. The coupling conditionis monitored by the network analyzer 31, which displays an impedancecurve on a Smith chart as shown in FIG. 6( a). By tuning thecapacitances of C1 and C2 in the match-up network 22 and the relativeposition between the specimen 4 and the heating head coupler 21, anoptimal condition can be reached when the nose of the impedance curve isbrought to the center of the Smith Chart as shown in FIG. 6( b). At thispoint, the RF/microwave system has been tuned to the best couplingcondition with minimum microwave reflection. The frequency marked at thenose point is the exact resonant frequency of the microwave system.

Processes of RF/Microwave Rapid Heating

Once the RF/microwave system is tuned to the optimal coupling conditionafter a small signal test, RF/microwave power at the system resonantfrequency can be applied to the target either in a form of a continuouswave (CW) or a series of pulsed waves. The target will be rapidly heatedup and held at the processing temperature for the required time. Thewhole process can be controlled either manually or electronically. Therunning time and RF/microwave power level for heating processes varieswith the requirements of applications and needs to be determined viaexperimental tests.

Since the resonant frequency and impedance may shift during the processdue to the temperature change of specimens, the coupling conditions maybe improved if the operating frequency can sweep over a narrow frequencyrange of ΔF during the heating period where ΔF is the frequency shiftingcaused by the temperature change.

Methods for Microwave RTP of Semiconductors and Materials

Microwave RTP for SiC Wide Bandgap Semiconductors

One embodiment of the present invention is to teach a method forRF/Microwave rapid thermal processing of SiC wide bandgap semiconductor.It is known that the temperature required for post-implant annealing ofSiC semiconductor ranging from 1500 to 2000° C., and a short annealingtime ranging from a few minutes to a few seconds. Microwave rapidheating tests for SiC wafers were performed using the RF/microwaveapparatus of this invention.

A SiC specimen of 6×6×0.4 mm sliced from a Cree 4H—SiC wafer was placedunder an inductive heading head. The size of the inductive coil insidethe heating head is about 8×8×5 mm which was designed to be very closeto the size of target specimen so high microwave coupling efficiency canbe achieved. From a small signal test, the resonant frequency of theloaded microwave system is measured as 402 MHz. The impedance matchbetween the load and source is accomplished by tuning the adjustablecapacitors in the match-up network with minimal microwave reflection.

An HP 8505A network analyzer is used as a signal generator as well asthe microwave measurement equipment. Once the microwave power is turnedon, the signal output of 402 MHz from the HP 8505A is amplified by aKalmus LA300UM power amplifier. The microwave power is transmitted tothe heating head through a directional coupler of Nader model 3020A. Thetransmitted and reflected microwave power are measured using two HP8481A RF probes and two HP 436A power meters. An Ircon 710C pyrometer isused for temperature measurement. All the measured microwave andtemperature parameters are inputted and stored on a PC using NI 4351 andVirtual Bench, both products of National Instruments.

The typical temperature profiles are shown in FIG. 7( a). The data forvariation of temperature with time and input microwave power are listedin Table 1 of FIG. 8. As can be seen from the data, it took about 1 and4 seconds for a SiC specimen to reach the temperatures of 1400° C. and2200° C. respectively with 150 W input power. The average heating ratevaries from 1400° C./sec to 550° C./sec depending on the targettemperature. This experiment demonstrated that the microwave system ofthis invention has the capability of reaching ultra-high temperature ofover 3000° C. within seconds. Such capability is crucial for RTP of ionimplanted SiC wafers in order to achieve low sheet resistance and smallsurface roughness.

Microwave RTP for Fabrication of Si-CMOS Ultra-Shallow Junction.

Another embodiment of the present invention is to teach a method forfabrication of Si-CMOS ultra-shallow junction (USJ) using theRF/microwave apparatus of this invention. It is known that fabricationof next generation Si nano-scale devices requires new RTP capabilitiesof ultra-high heating and cooling rates from 100° C./sec to well above1000° C./sec. Experiments for microwave rapid heating of As doped Siwafers have been performed using the microwave RTP apparatus of thisinvention. The sample material is a N type silicon wafer heavily dopedwith As impurities. It has very low resistivity of 0.001 to 0.004ohm-cm. The specimens of 6×6×0.4 mm were closely placed under theinductive heading head and various microwave powers at a frequency of406 MHz is applied to rapidly heat the As doped Si specimens. The set-upof microwave apparatus and the process method are very similar to thatfor rapid heating of SiC wafers. The typical temperature profiles areshown in FIG. 7( b). The data for variation of temperature with time andinput microwave power are listed in Table 2 of FIG. 8. It can be seenthat the heating rate of over 1000° C./second was achieved at powerlevel above 80 W and it only took one second to heat the Si wafer to itsmelting temperature of 1410° C. at power level of 150 W. These resultsdemonstrate that the microwave apparatus of the present invention iscapable of rapidly heating of Si wafer to near silicon meltingtemperature at ultra-high heating rate of over 1000° C./second. However,these experiments were carried out using a microwave source withcontinuous sine waveform which is relatively slow in transient powerrising and thus may limit the heating rate. The heating rate may besignificantly increased if a pulsed type of microwave source with thecapability to supply peak power at much higher levels can be used. Theapproaches in the direction of using high power pulsed microwave sourceshave great potential in meeting the challenges of RTP of next generationCMOS ultra-shallow junctions.

Method for Microwave Bonding and Sealing of Mems and IC Devices

Another objective of the present invention is to teach a new approach ofbonding and sealing of mems and IC devices using RF/microwave rapid andselective heating apparatus of this invention. This new approach willallow the use of bonding materials of high melting temperatures orchemical resistance such as noble metals for strong bonding and hermeticsealing of mems and IC devices. The new approach also can eliminate thesolder bumps in flip-chip and BGA bonding of IC devices to achievesignificantly higher interconnect density.

The basic idea of the new approach aims to provide high temperature in aconfined region of the bonding interface using RF/microwave rapid andselective heating so strong bonding can be achieved for bonding mems andIC components, while keeping the temperature low at wafer level so themicrostructures and microelectronics which are already there can bepreserved. The two key aspects for this new approach are: (1) heatingmust be selective so only the targeted area to be heated; and, (2)heating must be fast so there is no time for the heat to spread outsideof the targeted heating area. The RF/microwave apparatus of the presentinvention has the capability to perform both the two key functions.

Rapid and Selective Heating Tests for Heterostructure.

Another embodiment of the present invention is to perform rapid andselective heating tests for heterostructure of a gold thin film coatedsilicon wafer using the method and apparatus of the present invention.The basic mechanism of microwave selective heating is based thevariation of microwave absorption in a heterostructure, a structurewhich comprises of a strong microwave absorber and a poor microwaveabsorber. Microwaves will selectively heat up the strong microwaveabsorber while leaving only negligible heating effect on the materialsof low microwave absorption. Gold has very high electrical conductivityso it is a strong microwave absorber. The conductivity of siliconsubstrate is low so it is a weak microwave absorber.

Referring to FIG. 9, a 20 nm thin layer of Cr film 41 is first depositedon a 4″ Silicon wafer 40 followed by the deposition of the second 0.3 μmlayer of gold film 42. Both Cr and Au thin films are coated using PVDsputtering deposition technique. The gold coated wafer 40 is then slicedinto small square specimens 43 with a dimension of 8×8×0.4 mm.

Referring to FIG. 10, a gold coated silicon specimen 43 is closelyplaced under an inductive coil 24. The size of the coil 24 is about10×10×5 mm and was designed to be very close to the size of targetspecimen 41 so high microwave coupling efficiency can be achieved.Various microwave powers at a frequency of 906 MHz was applied torapidly heat the gold thin film coated Si specimen. The set-up ofmicrowave apparatus and the process method are very similar to that forrapid heating of SIC wafers except the power amplifier of Kamuls LA300UMwas replaced by a Decibel C903 power amplifier

Typical temperature profiles for microwave rapid heating of gold coatedsilicon substrates are illustrated in FIG. 11. The gold's meltingtemperature of 1064° C. was reached within 2-3 seconds with inputmicrowave power of 120-140 W. A heating rate of 300° to 800° C./secondis achieved. The detailed experimental data for the variation oftemperature with time and input microwave power are listed in Table 3 ofFIG. 12. The results of this rapid heating test demonstrates that themicrowave apparatus of the present invention is capable of rapid andselective heating of materials to high temperatures above 1000° C. witha high heating rate of 300 to 800° C./sec.

Microwave Bonding of Mems Components with Gold Interlayer

Another embodiment of the present invention is to perform bonding of twosilicon components of mems micro-pump with gold interlayer usingmicrowave rapid heating apparatus of this invention. FIG. 13( a)illustrates the cross section of a micro-pump which will be used forbio-medical applications. The micro-bump is fabricated by bonding threecomponents of silicon substrates A, B and C together. The properfunction of the micro-pump requires high bonding strength, hermeticsealing and corrosion resistance at the gray areas of the bondinginterface. Microwave rapid bonding tests were performed for componentsusing gold as the bonding material because of gold's excellent corrosionresistance.

Referring to FIG. 13( b), silicon wafers A and B are formed bymicro-machining techniques and sliced into small specimens in adimension of 6 mm×6 mm×0.4 mm. The grey areas represent the pattern ofbonding interface on which gold thin films are deposited by PVDsputtering and lithographic methods. The PVD deposition processes areschematically shown in FIG. 13( c). A 20 nm thin layer of Cr film isfirst deposited on the bonding areas, followed by the deposition of thesecond 0.3 μM layer of gold film. The two silicon substrates are stackedup with the gold coated areas face to face along with the alignment ofthe bonding areas as shown in FIG. 13( d). The two stacked substratesform a heterostructure where the gold is a strong microwave absorber andthe silicon substrate is a weak microwave absorber.

The RF/microwave rapid bonding process is very similar to that ofRF/microwave rapid and selective heating tests for gold thin film coatedsilicon wafers. A circular inductive coil 23 (shown in FIG. 4) with adiameter of 10 mm is placed on the top of the bonding area. Microwavepower of 898 MHz is coupled to the bonding target through the inductivecoil 23. Since the silicon substrate is low dielectric loss material anda weak microwave absorber, the input microwave power can penetratedthrough the top silicon substrate and dissipated in the thin gold layersin the middle of the sandwiched mems components through an induced eddycurrent. It takes about 3 to 6 seconds to firmly bond the two componentsA and B of the mems micro-pump with microwave power level of 120 to 140watts.

Solderless Flip-Chip and Ball Grid Array (BGA) Bonding of IC Chips forHigh Interconnect Density

Another objective of the present invention is to teach a way ofeliminating solder bumps in flip-chip and BGA bonding of IC chips forhigh interconnect density. As the demand on miniaturization andfunctionality increases, higher-density interconnection and highernumber of I/Os are needed to keep up with the device complexity. Thelarge space occupied by the metal solder bumps, as shown in FIG. 14( a),becomes a limiting factor for further shrinkage of the IC package.RF/microwave bonding can meet the challenge of high interconnect densityby directly bonding gold pads on chip to gold pads for pins withoutsolder balls. Since RF/microwave bonding is stronger than solderbonding, the size of pads can be reduced, which will result insignificantly higher interconnect density. For instance the ball size ofa traditional flip chip bond, as shown in FIG. 14( a), is about 200-1000μm, while for RF/microwave bonding, the size of the pads may be as smallas 20-30 μm, as shown in FIG. 14( b).

The process for RF/microwave solderless gold pad to gold pad directbonding is similar to that of mems metal bonding. An induction head canbe placed on the top of IC microchip and eddy currents will be inducedin the gold coated pads. The two pads will be bonded when the meltingtemperature of 1065° C. is reached. The heating rate must be very fastso the bonding can be completed in a very short time. Only the interfaceof the pads is melted while other parts of the IC microchip are stillcool and the microcircuits in the IC chip are unaffected.

The method and apparatus of the present invention may introduce severalnew opportunities, including: (1) much higher temperatures can beapplied to improve the bonding strength; (2) new bonding mechanisms suchas brazing and fusion bonding, and new bonding materials, such as noblemetals of Au, Ag, Cr, Ni, Pt and SiO₂ based glass can be used; (3) nopressure needs to be applied to the specimens during bonding which cansignificantly reduce the residual stress related damages; and, (4)bonding is not limited to the outside surface of the body. Microwavescan penetrate into non-conductive or low conductive materials to performbonding inside the body as demonstrated in the bonding of sandwichedmems components described above. (5) a new direct gold pad to gold padbonding method for solderless Flip-Chip and Ball Grid Array (BGA)Bonding of IC Chips to achieve significantly higher interconnect density

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingcurrent knowledge, readily modify and/or adapt for various applicationssuch specific embodiments without departing from the generic concept.Therefore, such adaptations and modifications should and are intended tobe comprehended within the meaning and range of equivalents of thedisclosed embodiments. It is to be understood that the phraseology ofterminology employed herein is for the purpose of description and not oflimitation.

1. A method for rapid heating of materials, the method comprising:providing an apparatus including a variable high frequency microwavepower source, a microwave heating head, and a measurement and controlsystem, wherein the microwave heating head includes a heating headcoupler for coupling microwaves to a material to be heated and anelectronic match-up network including inductive (L) components andcapacitive (C) components for tuning a resonant frequency and animpedance of the heating head coupler; selecting an appropriate heatinghead, based on a type of material to be heated, wherein the selecting anappropriate heating head includes: selecting an inductive coil as theheating head, when the type of material to be heated includes one ofhigh conductive thin films and semiconductors; and selecting twocapacitive electrodes as the heating head, when the type of material tobe heated includes one of dielectric material, glass and ceramics;placing the material to be heated at a position proximal to the selectedheating head, thus loading the heating head such that the material to beheated is maximally exposed to a microwave power from the heating head;electronically adjusting the microwave power source and the L and Cparameters of the match-up network so that a characteristic frequencyand an impedance of the loaded heating head are optimally matched with acharacteristic frequency and an impedance of the microwave power sourcesuch that the microwave power from the heating head is focused uponselected portions of the material to be heated; generating microwaveswithin the microwave power source; applying the microwave power to thematerial to be heated, thus heating the material; adjusting themicrowave power source and the heating head during heating of thematerial so as to compensate for any frequency and impedance shifting ofthe heating head caused by a temperature change in the material duringheating; and controlling a microwave power waveform to heat the materialto a predetermined heating temperature at a predetermined heating rateusing the measurement and control system; wherein the apparatus iscapable of achieving a heating rate of 10-2000° C./second for highconductive films and semiconductors and does not heat the material in amicrowave cavity; and wherein the microwave power source can sweep apredetermined bandwidth within a frequency range from 300 MHz to 20 GHz.2. The method of claim 1, wherein the materials to be heated areselected from the group consisting of semiconductors, high conductivefilms, dielectric materials, glass and ceramics.
 3. The method of claim1, wherein the materials to be heated are semiconductors selected fromthe group consisting of Silicon, Silicon Carbide and Gallium Nitride. 4.The method of claim 1, wherein the materials to be heated aresemiconductors selected from the group consisting of ion-implantationdoped Silicon, Silicon Carbide, Gallium Nitride and Silicon basedmetal-oxide-semiconductor ultra-shallow-junction.
 5. The method of claim1, wherein the materials to be heated are high conductive films selectedfrom the group of metallic thin films consisting of Gold, Silver,Copper, Nickel, Chromium and Platinum.
 6. The method of claim 1,wherein: the heating head coupler includes an inductive coil having anumber of turns, and two capacitive electrodes, the number of turns inthe inductive coil is one or less when the driving frequency is higherthan 500 MHz, the number of turns in the inductive coil is two or lesswhen the driving frequency is between 300 MHz and 500 MHz, and aneffective electric length of the induction coil is less than a quarterof a wavelength of an applied driving microwave frequency.
 7. The methodof claim 1, wherein: the electronic match-up network includes at leastone inductive component and two capacitive components, and the twocapacitive components are connected in the form of a split capacitornetwork and each capacitive component has a variable capacitance.
 8. Themethod of claim 1, wherein the placing the material to be heated at aposition proximal to the selected heating head further comprises:placing the material to be heated at a position where a magnetic fieldis maximized, when the selected heating head is the inductive coil; andplacing the material to be heated at a position where an electric fieldis maximized, when the selected heating head is the two capacitiveelectrodes.
 9. The method of claim 1, wherein the microwave power sourcecomprises a solid state electronics based microwave signal generator anda solid state electronics based microwave power amplifier, and wherein adriving frequency of the microwave power source is greater than 300 MHz.10. The method of claim 1, wherein the microwave power source is able togenerate a continuous waveform, a pulsed waveform, and a modulatedwaveform.
 11. The method of claim 1, wherein the measurement and controlsystem comprises: an optical temperature sensor for monitoring theheating temperature; microwave probes and power meters for detecting thetransmitted and reflected microwave power; and a network analyzer and aPC based control system for electronically controlling the heating ofthe material.
 12. The method of claim 7, wherein the adjusting themicrowave power source and the heating head further comprises:determining a resonant frequency and an impedance of the loaded heatinghead using a network analyzer included with the measurement and controlsystem; adjusting the frequency of the microwave power source andadjusting the inductive component and the capacitive components of thematch-up network in the heating head, so that the frequency andimpedance of the microwave power source matches, respectively, theresonant frequency and impedance of the loaded heating head.
 13. Themethod of claim 7, wherein the generating microwaves and the applyingthe microwave power to the material to be heated further comprise:generating microwave power at a predetermined resonant frequency of thematerial to be heated; applying the microwave power to the material tobe heated; adjusting a frequency of the microwave power source so as tocompensate for any resonant frequency shifting of the loaded heatinghead caused by a temperature change of the material during heating;adjusting the inductive component and the capacitive components of thematch-up network to compensate for any frequency and impedance shiftingof the loaded heating head caused by a temperature change of thematerial during heating; and heating the material to a predeterminedtemperature at a predetermined heating rate controlled by themeasurement and control system.
 14. The method of claim 13, wherein thecontrolling a microwave power waveform to heat the material to apredetermined temperature at a predetermined heating rate furthercomprises: Electronically modulating the microwave power waveform usinga PC based control system so as to control the heating rate and theheating temperature to meet the predetermined heating rate and heatingtemperature.
 15. The method of claim 13, wherein the controlling amicrowave power waveform to heat the material to a predeterminedtemperature at a predetermined heating rate further comprises: Manuallypulsing an output of the microwave power so as to control the heatingrate and the heating temperature to meet the predetermined heating rateand heating temperature.